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An algorithm is presented to fit precipitates in atom-probe tomographic data sets as equivalent ellipsoids. Unlike previous techniques, which measure only the radius of gyration, these ellipsoids retain the moments of inertia and principle axes of the original precipitate, preserving crystallographic orientational information. The algorithm is applied to study interconnected γ′-precipitates (L12) in the γ-matrix (face-centered cubic) of a Ni–Al–Cr alloy. The precipitates are found to coagulate along 〈110〉-type directions. [Copyright &y& Elsevier]

What determines the morphology of a decomposing alloy? Besides the well-established effect of the nucleation barrier, we demonstrate that, in a concentrated multicomponent Ni(Al,Cr) alloy, the details of the diffusion mechanism strongly affect the kinetic pathway of precipitation. Our argument is based on the combined use of atomic-scale observations, using three-dimensional atom-probe tomography (3D APT), lattice kinetic Monte Carlo simulations and the theory of diffusion. By an optimized choice of thermodynamic and kinetic parameters, we first reproduce the 3D APT observations, in particular the early-stage transient occurrence of coagulated precipitates. We then modify the kinetic correlations among the atomic fluxes in the simulation, without altering the thermodynamic driving force for phase separation, by changing the vacancy–solute interactions, resulting in a suppression of coagulation. Such changes can only be quantitatively accounted for with non-zero values for the off-diagonal terms of the Onsager matrix, at variance with classical models. [ABSTRACT FROM AUTHOR]

Abstract: In a Ni-5.2 Al-14.2 Cr at.% alloy with moderate solute supersaturations and a very small γ/γ′ lattice parameter misfit, the nanostructural and compositional pathways during γ′(L12) precipitation at 873K are investigated using atom-probe tomography, conventional transmission electron microscopy, and hardness measurements. Nucleation of high number densities (N v >1023 m−3) of solute-rich precipitates (mean radius=〈R〉=0.75nm), with a critical nucleus composition of Ni-18.3±0.9 Al-9.3±0.7 Cr at.%, initiates between 0.0833 and 0.167h. With increasing aging time (a) the solute concentrations decay in spheroidal precipitates (〈R〉<10nm); (b) the observed early-stage coalescence peaks at maximum N v in coincidence with the smallest interprecipitate spacing; and (c) the reaction enters a quasi-stationary regime where growth and coarsening operate concomitantly. During this quasi-stationary regime, the γ (face-centered cubic)-matrix solute supersaturations decay with a power-law dependence of about −1/3, while the dependencies of 〈R〉 and N v are 0.29±0.05 and −0.64±0.06 at a coarsening rate slower than model predications. Coarsening models allow both equilibrium phase compositions to be determined from the compositional measurements. The observed early-stage coalescence is discussed in further detail. [Copyright &y& Elsevier]

Abstract: The kinetic pathways resulting from the formation of coherent γ′-precipitates from the γ-matrix are studied for two Ni–Al–Cr alloys with similar γ′-precipitate volume fractions at 873K. The details of the phase decompositions of Ni–7.5Al–8.5Cr at.% and Ni–5.2Al–14.2Cr at.% for aging times from 1/6 to 1024h are investigated by atom-probe tomography, and are found to differ significantly from a mean-field description of coarsening. The morphologies of the γ′-precipitates of the alloys are similar, though the degrees of γ′-precipitate coagulation and coalescence differ. Quantification within the framework of classical nucleation theory reveals that differences in the chemical driving forces for phase decomposition result in differences in the nucleation behavior of the two alloys. The temporal evolution of the γ′-precipitate average radii and the γ-matrix supersaturations follow the predictions of classical coarsening models. The compositional trajectories of the γ-matrix phases of the alloys are found to follow approximately the equilibrium tie-lines, while the trajectories of the γ′-precipitates do not, resulting in significant differences in the partitioning ratios of the solute elements. [Copyright &y& Elsevier]